This invention relates generally to phase synchronization of a radio frequency signal and specifically to unambiguous phase synchronization of a digitally modulated radio frequency signal.
Many digital communication systems employ synchronous detection of a radio frequency carrier transmitted from a transmitter to a receiver. These systems require the received signal to be phase synchronized to the transmitted signal in order for synchronous detection to properly occur. However, conventional synchronization techniques result in phase ambiguity.
Phase ambiguity is caused by the receiver acquiring a transmitted carrier signal but not knowing the orientation or phase of that signal in relation to the signal as transmitted. One example of phase ambiguity occurs in conventional quadrature amplitude modulated (QAM) signaling. A QAM signal can be thought of as a constellation of points arrayed in a phase plane with a real and imaginary axis. The transmitted QAM signal is in phase alignment with the received QAM signal when the axes defining the phase plane of the received QAM signal are oriented and aligned the same way as the transmitted plane""s axes. A phase ambiguity results when the receiver acts as if phase alignment has been achieved, but in actuality the received signal is inverted and 180 degrees out of phase (or some other angle than zero degrees).
Effects of phase ambiguity can be demonstrated in the example of two phase ambiguity that occurs in many systems. A received value may be either a binary xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d. The signal corresponding to a xe2x80x9c1xe2x80x9d is 180 degrees out of phase with the signal corresponding to a xe2x80x9c0xe2x80x9d, such that when the phase synchronization of the system is 180 degrees out of phase, data will be inverted. Until the ambiguity is resolved, the receiver is able to detect a symbol, but does not know if the received symbol is a xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
One attempt at solving the problem of phase ambiguity includes differential detection techniques. In a traditional differential detection implementation, the receiver contains circuitry to split the signal and recombine a delayed version of the signal with the original signal. However, differential detection techniques result in additional signal errors due to the delayed versions of original signal errors.
Another attempt at solving the phase ambiguity problem includes the use of a pilot carrier. This approach reduces phase ambiguity inherent in synchronous detection by using a reference carrier having a phase offset of ninety degrees. The pilot carrier approach has the advantage of achieving phase synchronization and eliminating phase ambiguity. However, this technique requires costly additional circuitry to generate the reference carrier and assure that the reference carrier is ninety degrees out of phase with the RF carrier. Further, additional power is needed to generate the reference carrier resulting in a decrease in power efficiency of the system.
Therefore, there exists a need for a method, apparatus, and system for efficiently and inexpensively acquiring unambiguous phase synchronization of a RF carrier in a digital communication system.